1. Field of the Invention
The present invention relates to a semiconductor laser element and a method of fabricating the same, and more particularly to a AlGaAs-based ridge-stripe semiconductor laser element withlow operational voltage and low element resistance, and a method of fabricating the element.
2. Description of the Related Art
In recent years, AlGaAs-based infrared wavelength semiconductor laser elements are used as light sources for reading devices, rewriting devices and initializers for optical disks. In particular, infrared high-output semiconductor laser elements of a broad-stripe type capable of oscillating lateral multi-mode laser light are expected as light sources for exciting solid lasers such as Nd:YAG and Nd:YVO4 having absorption band of the crystals at around 808 nm. Also processing tools such as welders based on use of the infrared high-output semiconductor lasers are becoming increasingly popular.
In these fields, the high-output semiconductor laser elements have particularly been on demand for realizing a still higher light confinement efficiency and a still lower threshold current.
Referring to FIG. 10, a configuration of a conventional AlGaAs-base semiconductor laser element having a buried-ridge-type structure will be described. FIG. 10 is a cross-sectional view of a conventional AlGaAs-base semiconductor laser element having the buried-ridge-type structure.
As shown in FIG. 10, a conventional buried-ridge-type AlGaAs-base semiconductor laser element 200 includes a double-hetero-stacked structure formed on an n-GaAs substrate 201, where the stacked structure comprises an n-GaAs buffer layer 202, an n-Al0.47Ga0.53As cladding layer 203, an active layer section 204, a p-Al0.47Ga0.53As first cladding layer 205, an etching stop layer 206, a p-Al0.47Ga0.53As second cladding layer 207 and a p-GaAs contact layer 208, all of which are epitaxially grown sequentially in this order.
The p-second cladding layer 207 and p-contact layer 208 are formed as a ridge so as to constitute a current injection region 220. Both lateral sections of the ridge composing the current injection region 220 are filled with an n-GaAs current blocking layer 211 to thereby form non-current-injection regions 221.
A p-side electrode 212 is formed on the upper surfaces of the p-contact layer 208 and the n-GaAs current blocking layer 211, and an n-side electrode 213 is formed on the back surface of the n-GaAs substrate 201.
In the semiconductor laser element 200, filling of both lateral sections of the ridge-stripe-patterned current injection region 220 with a semiconductor material of a conduction type opposite to the current injection region 220 is successful in realizing narrowing of both the current and refractive-index-based waveguide at the same time.
It can thus be said that the aforementioned semiconductor laser element 200 has a configuration capable of effectively confining both injected carriers and laser light.
The following paragraphs will describe a method of fabricating the conventional semiconductor laser element 200 referring to FIG. 11A to FIG. 13F. FIGS. 11A and 11B, FIGS. 12C and 12D, and FIGS. 13E and 13F are cross-sectional views showing layer structures in the individual process steps in the fabrication of the conventional semiconductor laser element 200.
First, as shown in FIG. 11A, the n-GaAs buffer layer 202, the n-Al0.47Ga0.53As cladding layer 203, the active layer section 204, the p-Al0.47Ga0.53As first cladding layer 205, the etching stop layer 206, the p-Al0.47Ga0.53As second cladding layer 207, and the p-GaAs contact layer 208 are epitaxially grown sequentially in this order on the n-GaAs substrate 201 in the first epitaxial growth step by an organometallic vapor phase growth process such as the MOVPE process and MOCVD process, to thereby form a stacked structure 210 having a double hetero-structure.
In the epitaxial growth, Si, Se and so forth are used as the n-type dopant, and Zn, Mg, Be and so forth as the p-type dopant.
Next, as shown in FIG. 11B, an SiO2 film 214 is formed on the top surface of the stacked structure 210, that is, the upper surface of the p-GaAs contact layer 208, by a CVD (Chemical Vapor Deposition) process or the like, and further on the SiO2 film 214, a stripe-patterned resist mask 215 is formed by photolithography.
Next, the SiO2 film 214 is mask-patterned with the resist mask 215, and the resist mask 215 is then removed, to thereby form an SiO2 mask 214 on the p-GaAs contact layer 208, as shown in FIG. 12C.
Next, the p-GaAs contact layer 208 and the p-Al0.47Ga0.53As second cladding layer 207 are etched by wet etching technique under masking with the SiO2 mask 214, to thereby form a ridge.
The etching is carried out using an etchant which is capable of completely removing the p-GaAs contact layer 208 and the p-Al0.47Ga0.53As second cladding layer 207, and having an etching selectivity enough to terminate the etching on the surface of the etching stop layer 206. This makes it possible to selectively remove the p-Al0.47Ga0.53As second cladding layer 207 without affecting the etching stop layer 206.
Next, as shown in FIG. 13E, the process advances to a second epitaxial step, where the n-GaAs current blocking layer 211 is grown on both lateral potions of the ridge. Because the SiO2 mask 214 resides on the ridge, the GaAs current blocking layer 211 does not grow on the ridge.
Next as shown in FIG. 13F, the SiO2 mask 214 is removed, the p-side electrode 212 is formed on the p-contact layer 208 and the n-GaAs current blocking layer 211, and the n-side electrode 213 is formed on the back surface of the n-GaAs substrate 201.
The aforementioned conventional semiconductor laser element 200 is obtained after all of the above-mentioned process steps.
The above-described semiconductor laser element 200, however, presents the following disadvantages as described below:
a first disadvantage is that the semiconductor laser element has a high operational voltage and high element resistance due to structural reasons; and
a second drawback is that the fabrication processes involved are complicated and are costly again due to structural reasons.
A typical problem arises in association with the ridge formation. Assumption is made now that, in the process shown in FIG. 12D, the etching stop layer 206 is composed of AlmGa1-mAs, and a hydrofluoric-acid-containing etchant is used in the etching for forming the stripe-patterned ridge.
Assuming now that the etching stop layer 206 composed of AlmGa1-mAs has no etching selectivity against the hydrofluoric-acid-containing etchant, the etching may proceed so as to penetrate the etching stop layer 206 to reach the p-Al0.47Ga0.53As first cladding layer 205, and may even reach the active layer section 204 depending on occasions.
Because the etchrate attainable by the hydrofluoric-acid-containing etchant depends on the Al compositional ratio, it is found necessary to set the Al compositional ratio “m” so as to allow the etching to proceed through the p-Al0.47Ga0.53As second cladding layer 207, but to terminate on the etching stop layer 206, so far as the hydrofluoric-acid-containing etchant is adopted.
Lowering the Al compositional ratio “m” so as to expand the etching selectivity between the p-Al0.47Ga0.53As second cladding layer 207 and the etching stop layer 206, for example, undesirably raises a problem of increase in the element resistivity due to increase in the carrier recombination within the etching stop layer 206.
In place of the adjustment of the Al compositional ratio “m” of the etching stop layer 206, another possible strategy is such as raising the concentration of the hydrofluoric-acid-containing etchant aiming at increasing the etching selectivity between the p-Al0.47Ga0.53As second cladding layer 207 and the etching stop layer 206. This, however, raises another problem of lowering in the etchrate of the p-Al0.47Ga0.53As second cladding layer 207, which inhibits the etching.
Therefore, one cannot help saying that it is difficult to control the etching selectivity based on concentration of the hydrofluoric-acid-containing etchant.
Another problem of the hydrofluoric-acid-containing etchant resides in that it can etch also the SiO2 mask 214. It is therefore necessary to determine the concentration of the hydrofluoric-acid-containing etchant so as to ensure the etching selectivity against the SiO2 mask 214, and it is still also necessary to adjust the thickness of the SiO2 mask 214. In short, use of the hydrofluoric-acid-containing etchant is labor-consuming.
There is proposed an alternative method in which GaInP is used for composing the etching stop layer 206, and a sulfuric-acid-containing etchant is adopted as the etchant in place of the hydrofluoric-acid-containing etchant, which may successfully increase the etching selectivity ratio, and thereby terminate the etching on the surface of the etching stop layer 206.
Adoption of GaInP for the etching stop layer 206, however, inevitably requires replacement of the previous As-containing furnace atmosphere with a P-containing furnace atmosphere, and lowering of the growth temperature in the furnace, in order to grow the GaInP etching stop layer in the first epitaxial growth step. After the GaInP etching stop layer was grown, the growth temperature in the furnace must be elevated again, the As-containing atmosphere must be recovered in order to grow the residual p-Al0.47Ga0.53As second cladding layer 207, and the p-GaAs contact layer 208.
This makes the crystal growth process more complex, extends the operation time in the epitaxial growth process, and raises the costs.
It is also known that a high-output, broad-stripe-type semiconductor laser element using the GaInP etching stop layer may largely vary the geometry of the NFP (Near Field Pattern) as being affected by lattice distortion induced by the GaInP etching stop layer.
In view of such problems, Japanese Laid-Open Patent Publication No. 5-259574 proposes a method of forming the ridge by selectively etching the cladding layer using an etchant comprising an organic acid and hydrogen peroxide.
In other words, the patent publication describes that use of AlGaAs having an Al compositional ratio of 0.38 to 0.6 for the cladding layer, use of an AlGaAs layer having an Al compositional ratio of 0.6 or larger for the etching stop layer, and use of a specified etchant is successful in forming the ridge with a good reproducibility, and consequently in readily fabricating the semiconductor laser element.
The above-described patent publication discloses an example in which the cladding layer is composed of Al0.5Ga0.5As, a 0.06-μm-thick etching stop layer is composed of Al0.6Ga0.4As, and the specific etchant comprises a mixed solution of tartaric acid and an aqueous hydrogen peroxide solution.
In addition, this specific etchant can etch the Al0.5Ga0.5As layer but cannot etch the Al0.6Ga0.4As layer, so that the etching terminates upon exposure of the Al0.6Ga0.4As layer, and this ensures the ridge formation with good reproducibility.
The patent publication also describes that use of the same AlGaAs layer both for the etching stop layer and the cladding layer makes it possible to grow the etching stop layer and the cladding layer under same growth conditions, so that only a control of the Al compositional ratio is successful in readily forming the cladding layer and the etching stop layer in a succeeding manner with an advantageous crystallinity.
It is also described that the light confinement efficiency can be raised because the etching stop layer is adjusted to have an Al compositional ratio larger than that of the cladding layer, and this makes it possible to provide a region having a refractive index smaller than that of the cladding layer.
The configuration of the semiconductor laser element disclosed in the patent publication, however, has the Al0.6Ga0.4As etching stop layer having a refractive index smaller than that of the Al0.5Ga0.5As cladding layer on the ridge side, and this results in a problem that the light generated in the active layer is undesirably pushed out towards the opposite side of the ridge, and consequently makes it difficult to raise the light confinement efficiency.
Another problem resides in that increase in the thickness of the Al0.6Ga0.4As etching stop layer may be successful in improving the optical characteristics, but the Al0.6Ga0.4As etching stop layer having a band gap energy larger than that of Al0.5Ga0.5As composing the cladding layer also serves as a barrier against the carriers, and increase in the thickness thereof may result in increase in the threshold current.